
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>jquery</title>
</head>
<body>
    <h1>jquery</h1>
    <div id="app">
        
    </div>
    <script src="./demo.js"></script>
    <script>
        console.log(jQuery);
        console.log($);
        $('#app').add()
        $('#app').remove()

    </script>
</body>
</html>